Symptoms
Consider the following scenario:
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You have an Exchange Server hybrid configuration.
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Exchange Server on-premises doesn't have direct outbound connectivity to Exchange Online (EXO). You have a proxy server that's configured to enable outbound connectivity to EXO.
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You enable archive mailboxes that are hosted in EXO.
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You use retention policies and tags to manage elements in your users’ mailboxes.
In this scenario, Exchange Server doesn't process the elements that are hosted in the archive mailbox. Exchange Server stops responding and returns the following exception:
System.Net.WebException: Unable to connect to the remote server
When this problem occurs, Exchange Server ignores the configured proxy server and tries to connect to EXO directly.
Resolution
To fix this issue, install the following cumulative update:
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Cumulative Update 15 for Exchange Server 2019 or a later cumulative update for Exchange Server 2019
